1. Field of the Invention
The invention relates to a method and an apparatus for preheating the fuel for a firing plant which is operated with liquid or gaseous fuel, which is fed to the firing plant via a feed line, having at least one heat-exchanger unit which is thermally coupled to the feed line and through which heat is transferred from a heating medium, which is directed in a heating circuit, to the fuel.
2. Discussion of Background
Firing plants which have a burner unit and serve by means of specific feeding and combustion of gaseous and/or liquid fuel to heat a working medium, which in turn is utilized to generate energy or perform work, are improved by constant reworking and design optimization from the point of view of increasing the efficiency and/or the output of the combustion process.
DE 43 21 081 A1 discloses a typical representative of firing plants, namely a gas-turbine plant, having a combustion chamber, in which fuel is specifically introduced for the subsequent combustion and formation of hot gases driving the gas turbine. To increase the efficiency of the gas turbine and in particular of the combustion process, it is proposed to preheat the fuel to be directed into the combustion chamber to temperatures of between 100.degree. C. and 400.degree. C. To this end, a heat exchanger is provided in the fuel feed line. In this heat exchanger, a heating medium gives off heat to the fuel flowing in the fuel line.
The plant described in the abovementioned publication is in particular a combined gas/steam-turbine plant in which the flue gases issuing from the gas turbine are directed via an exhaust-gas line into a heat-recovery steam generator, the waste heat of which serves to preheat and deaerate the feedwater and to generate steam in a low-pressure stage and a high-pressure stage. Low-pressure steam and high-pressure steam are used in a steam turboset to generate electrical energy. The exhaust steam is precipitated in the condenser and the condensate is fed again to the heat-recovery steam generator.
For the preheating of the fuel fed in the fuel line of the gas turbine to the aforesaid desired temperature level, it is advisable to connect up the heat exchanger integrated in the fuel line with a feed line for the heating medium from the feedwater tank/deaerator and with a discharge line into the condensate line.
An essential aspect of the preheating of the fuel mentioned in the publication relates to the avoidance of the ingress of gaseous constituents of the fuel into the heating circuit, as a result of which the heating medium may be contaminated. Such contamination, which is relevant from the safety point of view, may be caused in particular by leakages in the heating areas. For this reason, the pressure on the side of the heating medium must be greater than the pressure of the fuel. Thus the ingress of fuel into the heating circuit can be ruled out. If any leakages occur, only a discharge of heating medium from the heating circuit can take place. To ensure the necessary pressure ratios, a pump which increases the pressure upstream of the heat exchanger is provided in the feed line of the heating medium to the heat exchanger. On the other side, a corresponding choke unit is provided in the discharge line, which choke unit readjusts the increased pressure to the conditions in the condensate line.
A disadvantage in this circuit variant is the pressure increase by means of a pump with subsequent pressure reduction in a choke unit.
An alternative variant provides for the heating medium to be bled from the outlet of the high-pressure economizer in order to ensure in this way that the pressure prevailing in the feed line of the heating medium is greater than the pressure to which the fuel in the fuel line is subjected. The heating medium is returned into the feedwater tank/deaerator and this return requires a corresponding choke device.
For energy reasons, the variant, described above, of the bleeding of the heating medium from the high-pressure stage of the heat-recovery boiler is not especially advantageous, since the high energy level, predetermined by pressure and temperature, of the heating medium should be utilized inside the steam turbine in order to generate energy and is not available for another use at a low energy level, such as, for example, for the preheating of the fuel.